The use of magnetic field measurement devices (e.g., magnetometers) in prior art subterranean surveying techniques for determining the direction of the earth's magnetic field at a particular point is well known. The use of accelerometers or gyroscopes in combination with one or more magnetometers to determine direction is also known. Deployments of such sensor sets are well known, for example, to determine borehole characteristics such as inclination, borehole azimuth, positions in space, tool face rotation, magnetic tool face, and magnetic azimuth (i.e., the local direction in which the borehole is pointing relative to magnetic north). Moreover, techniques are also known for using magnetic field measurements to locate magnetic subterranean structures, such as a nearby cased borehole (also referred to herein as a target well). For example, such techniques are sometimes used to help determine the location of a target well, for example, to reduce the risk of collision and/or to place the well into a kill zone (e.g., near a well blow out where formation fluid is escaping to an adjacent well).
The magnetic techniques used to sense a target well may generally be divided into two main groups; (i) active ranging and (ii) passive ranging. In active ranging, the local subterranean environment is provided with an external magnetic field, for example, via a strong electromagnetic source in the target well. The properties of the external field are assumed to vary in a known manner with distance and direction from the source and thus in some applications may be used to determine the location of the target well. The use of certain active ranging techniques, and limitations thereof, in twin well drilling is discussed in more detail below.
In contrast to active ranging, passive ranging techniques utilize a preexisting magnetic field emanating from magnetized components within the target borehole. In particular, conventional passive ranging techniques generally take advantage of remanent magnetization in the target well casing string. Such remanent magnetization is typically residual in the casing string because of magnetic particle inspection techniques that are commonly utilized to inspect the threaded ends of individual casing tubulars.
Various passive ranging techniques have been developed in the prior art to make use of the aforementioned remanent magnetization of the target well casing string. For example, as early as 1971, Robinson et al., in U.S. Pat. No. 3,725,777, disclosed a method for locating a cased borehole having remanent magnetization. Likewise, Morris et al., in U.S. Pat. No. 4,072,200, and Kuckes, in U.S. Pat. No. 5,512,830, also disclose methods for locating cased boreholes having remanent magnetization. These prior art methods are similar in that each includes making numerous magnetic field measurements along the longitudinal axis of an uncased (measured) borehole. For example, Kuckes assumes that the magnetic field about the target well varies sinusoidally along the longitudinal axis thereof. Fourier analysis techniques are then utilized to determine axial and radial Fourier amplitudes and the phase relationships thereof, which may be processed to compute bearing and range (direction and distance) to the target borehole. Moreover, each of the above prior art passive ranging methods makes use of the magnetic field strength and/or a gradient of the magnetic field strength to compute a distance to the target well. For example, Morris et al. utilize measured magnetic field strengths at three or more locations to compute gradients of the magnetic field strength along the measured borehole. The magnetic field strengths and gradients thereof are then processed in combination with a theoretical model of the magnetic field about the target well to compute a distance between the measured and target wells.
While the above mentioned passive ranging techniques attempt to utilize the remanent magnetization in the target well, and thus advantageously do not require positioning an active magnetic or electromagnetic source in the target borehole, there are drawbacks in their use. For example, the magnetic field strength and pattern resulting from the remanent magnetization of the casing string tubulars is inherently unpredictable for a number of reasons. First, the remanent magnetization of the target borehole casing results from magnetic particle inspection of the threaded ends of the casing tubulars. This produces a highly localized magnetic field at the ends of the casing tubulars, and consequently at the casing joints within the target borehole. Between casing joints, the remanent magnetic field may be so weak that it cannot be detected reliably. A second cause of the unpredictable nature of the remanent magnetism is related to handling and storage of the magnetized tubulars. For example, the strength of the magnetic fields around the ends of the tubulars may change as a result of interaction with other magnetized ends during storage of the tubulars prior to deployment in the target borehole (e.g., in a pile at a job site). Finally, the magnetization used for magnetic particle inspection is not carefully controlled because the specific strength of the magnetic field imposed is not important. As long as the process produces a strong enough field to facilitate the inspection process, the field strength is sufficient. The resulting field can, therefore, vary from one set of tubulars to another. These variations cannot be quantified or predicted because no record is generally maintained of the magnetization process used in magnetic particle inspection.
Consistent with the above, the Applicant has observed that the magnetic pole strength may vary from one wellbore tubular to the next by a factor of 10 or more. Moreover, the magnetic poles may be distributed randomly within the casing string, resulting in a highly unpredictable magnetic field about the target well. As such, determining distance from magnetic field strength measurements and/or gradients of the magnetic field strength is problematic. A related drawback of prior art passive ranging methods that rely on the gradient of the residual magnetic field strength is that measurement of the gradient tends to be inherently error prone, in particular in regions in which the residual magnetic field strength of the casing is small relative to the local strength of the earth's magnetic field. Reliance on such a gradient may cause errors in calculated distance between the measured and target wells.
McElhinney, in co-pending, commonly assigned U.S. patent application Ser. No. 10/705,562, discloses a passive ranging methodology, for use in well twinning applications, in which two-dimensional magnetic interference vectors are typically sufficient to determine both the bearing and range to the target well. The two-dimensional interference vectors are utilized to determine a tool face to target angle (i.e., the direction) to the target well, e.g., relative to the high side of the measured well. The tool face to target angles at first and second longitudinal positions in the measured well may also be utilized to determine distance to the target well. The McElhinney disclosure addresses certain drawbacks with the prior art in that neither the strength of the remanent magnetic field nor gradients thereof are required to determine distance. Moreover, the bearing and range to the target well may be determined at a single survey station for a downhole tool having first and second longitudinally spaced magnetic field sensors.
While the above described McElhinney technique and other passive ranging techniques have been successfully utilized in commercial well twinning applications, their effectiveness is limited in certain applications. For example, passive ranging techniques are limited by the relatively weak remanent magnetic field about the target well and by the variability of such fields. At greater distances (e.g., greater than about 4 to 6 meters) a weak or inconsistent magnetic field about the target well reduces the accuracy and reliability of passive ranging techniques. Even at relatively smaller distances there are sometimes local regions about the target well where the remanent magnetic field is too weak to make accurate range and bearing measurements. Active ranging techniques, on the other hand, produce a more consistent and predictable field around the target borehole. For this reason active ranging techniques have been historically utilized for many well twinning applications.
For example, active ranging techniques are commonly utilized in the drilling of twin wells for steam assisted gravity drainage (SAGD) applications. In such SAGD applications, twin horizontal wells having a vertical separation distance typically in the range from about 4 to about 20 meters are drilled. Steam is injected into the upper well to heat the tar sand. The heated heavy oil contained in the tar sand and condensed steam are then recovered from the lower well. The success of such heavy oil recovery techniques is often dependent upon producing precisely positioned twin wells having a predetermined relative spacing in the horizontal injection/production zone (which often extends up to and beyond 1500 meters in length). Positioning the wells either too close or too far apart may severely limit production, or even result in no production, from the lower well.
Prior art methods utilized in drilling such wells are shown on FIGS. 1A and 1B. In each prior art method, the lower production well 30 is drilled first, e.g., near the bottom of the oil-bearing formation, using conventional directional drilling and measurement while drilling (MWD) techniques. In the method shown on FIG. 1A, a high strength electromagnet 34 is pulled down through the cased target well 30 via tractor 32 during drilling of the upper well 20. An MWD tool 26 deployed in the drill string 24 near drill bit 22 measures the magnitude and direction of the magnetic field during drilling of the upper well 20. In the method shown on FIG. 1B, a magnet 27 is mounted on a rotating collar portion of drilling motor 28 deployed in upper well 20. A wireline MWD tool 36 is pulled (via tractor 32) down through the cased target well 30 and measures the magnitude and direction of the magnetic field during drilling of the upper well 20. Both methods utilize the magnetic field measurements (made in the upper well 20 in the approach shown on FIG. 1A and made in the lower well 30 in the approach shown on FIG. 1B) to compute a range and bearing from the upper well 20 to the lower well 30 and to guide continued drilling of the upper well 20.
The prior art active ranging methods described above, while utilized commercially, are known to include several significant drawbacks. First, such methods require simultaneous and continuous access to both the upper 20 and lower 30 wells. As such, the wells must be started a significant distance from one another at the surface. Moreover, continuous, simultaneous access to both wells tends to be labor and equipment intensive (and therefore expensive) and can also present safety concerns. Second, the remanent magnetization of the casing string (which is inherently unpredictable as described above) is known to sometimes interfere with the magnetic field generated by the electromagnetic source (electromagnet 34 on FIG. 1A and magnet 27 on FIG. 1B). While this problem may be overcome, (e.g., in the method shown on FIG. 1A magnetic field measurements are made at both positive and negative electromagnetic source polarities), it is typically at the expense of increased surveying time, and thus an increase in the time and expense required to drill the upper well. Third, the above described prior art active ranging methods require precise lateral alignment between the magnetic source deployed in one well and the magnetic sensors deployed in the other. Misalignment can result in a misplaced upper well, which as described above may have a significant negative effect on productivity of the lower well. Moreover, the steps taken to assure proper alignment (such as making magnetic field measurements at multiple longitudinal positions in one of the wells) are time consuming (and therefore expensive) and may further be problematic in deep wells. Fourth, a downhole tractor 32 is often required to pull the magnetic source 34 (or sensor 36 on FIG. 1B) down through the lower well 30. In order to accommodate such tractors 32, the lower well 30 must have a sufficiently large diameter (e.g., on the order of 12 inches or more). Thus, elimination of the tractor 32 may advantageously enable the use of more cost effective, smaller diameter (e.g., seven inch) production wells. Moreover, in a few instances, such downhole tractors 32 have been known to become irretrievably lodged in the lower well 30.
Therefore, there exists a need for improved magnetic ranging methods suitable for twin well drilling (such as twin well drilling for the above described SAGD applications). In particular, there exists a need for a magnetic ranging technique that combines advantages of active ranging and passive ranging techniques without inheriting disadvantages thereof.